1. Field of the Invention
This invention is concerned with an improved liquid developer for developing electrostatic latent images in electrostatic printing, electrophotography, and so forth.
2. Description of the Prior Art
Various liquid developers which have so far been used in developing electrostatic latent images are of such type that an additive like resin, etc. is dispersed in a highly insulative carrier liquid. The additive serves to control the content of a pigment and charged polarity of the pigment so as to facilitate dispersion of the pigment into the carrier liquid, and at the same time, serves to secure good fixation of a tone to a developed image.
Such a liquid developer for electrostatic latent images can be prepared by, as one example, mixing and kneading of a pigment such as carbon black with vegetable oils, resins, and so on, e.g., linseed oil, soybean oil, alkyd resin, polystyrene, acrylic resins, etc. as the polarity controlling agent, dispersing agent as well as fixing agent, followed by granulation of the kneaded mixture, and then by dispersing the particles into a highly insulative organic solvent such as hydrocarbons of the isoparaffin type.
Another known method of controlling the polarity of the toner in the liquid developer for electrostatic latent images other than coating the outer surface of the toner particles with vegetable oils, resins, etc. as mentioned above, includes dissolving a surface active agent in the carrier liquid so as to cause it to absorb onto the toner particles.
Many examples of the surface active agent to be used in the abovementioned second method are known, such as heavy metal soaps like cobalt naphthenate, manganese naphthenate, etc.; alkylbenzene sulfonic acid salts like dodecylbenzene calcium sulfonate, etc.; phospholipide (or phosphatides) as lecithin, cephalin, etc. These surface active agents, however, are not necessarily consistent in their resulting effect and manners of use. Moreover, as they are of low electric resistance, they inevitably lower the electric resistance of the carrier liquid when they are dissolved in such carrier liquid. In this consequence, the addition of such a surface active agent to the carrier liquid should be done to such an extent that it does not destroy the latent images formed on a photosensitive body at the time of the image development, on account of which the adding quantity of the surface active agent to the carrier liquid undergoes considerable restriction. In view of such restriction, the aforedescribed method is considered defective in that no sufficient amount of the surface active agent to give satisfactory electric charging quantity to the toner can be added to the carrier liquid.
With respect to liquid developers for use in color electrophotography, the conventional color electrophotographic method includes first imparting an electric charge to a photosensitive body having a photoconductive layer by corona discharge, thereafter resolving the original picture image into color components by means of color filters of red, blue, and green to thereby form an electrostatic latent image on the photosensitive body, and then immersing the electrostatic latent image in each of the developing liquids containing therein cyan, magenta, yellow, and black toners to develop and reproduce the color image.
In the above-described color process, the multi-colored image is formed on the same photosensitive body in succession. Then the latent image formed on the photosensitive body is developed and reproduced into the colored picture image like the original. If the photosensitive body having thereon the already toned image is immersed in another developing liquid for a different color, then the toner composition of resins, polarity controlling agent, etc. elute, or the toner itself can dissolve into the developing liquid to contaminate it also, at the time of superposing the developed images one by one, non-uniformity is trapping due to the light shielding effect of the prior toned image, and other disadvantages, tend to take place.
In obtaining a color reproduction image by transferring a colored picture image deposited on the photoconductive layer onto a transfer sheet one after another, the trouble caused by the light shielding effect of the toned picture image can be advantageously prevented. However, for the purpose of this color electrophotography, there has not been available an appropriate color toner, the fixation of which is suited for the transfer process, and, if such is used, no uniform transfer of the toner onto the transfer sheet can be attained to a required standard.
Further, known liquid developers for color electrophotography, the substance added for imparting to the pigment charge polarity, the dispersibility, and the fixing property is a vegetable oil such as linseed oil, soybean oil, etc., and a resin such as acrylic resin, rosin, etc. On account of this, when the pigment particles are coated with any of these coating materials, and dispersed in the carrier liquid, a large part of the coating agent coated on the pigment dissolves out into the carrier liquid and the residual quantity of the coating agent on the surface of the pigment becomes extremely small, with the consequence that the toner becomes less fixable to the recorded image, also, toner which has once adhered to the surface of the recorded image dissolves into the carrier liquid to contaminate the developing liquid. For this reason, the coating agent tends to separate gradually from the external surface of the pigment during repeated use of the developing agent or storage for long periods to thereby obscure the polarity of the toner, or the toner tends to become agglomerated and precipitated to make it impossible to reproduce the desired color.
Furthermore, in the case of a color reproduction, a desired color is reproduced by superimposing picture images of several color phases. If the resin coated on the toner particles separates therefrom for the above mentioned reason, the image loses its luster, and the depth of the colors of the image and the mixing property of the colored picture image are in no way sufficient. As a means to supplement this defect, it has been unavoidable to coat the surface of the developed image with a layer of a resinous material.